There is a growing demand for affordable mid-infrared sources for use in a variety of applications including eye-safe medical laser sources for non-invasive medical diagnostics, eye-safe laser radar and remote sensing of atmospheric constituents, optical communication, and numerous military applications. These applications rely on the existence of “spectroscopic fingerprints” of numerous organic molecules in the mid-IR range of optical spectrum.
Such organic molecules play the fundamental role in numerous biochemical processes on a single cell level as well as in most other fields of life sciences, in environmental research, in industrial processes, and in global geo-physical processes. Hence, it is essential to detect the molecular content of a given sample with full chemical specificity, with ultimate sensitivity, and with real-time speed. For example, one such area where measuring a gas sample may be effective in addressing disease early is in identifying biomarkers for cancer. More specifically, lung cancer is the most common cause of cancer-related mortality in both men and women in the United States. An estimated 173,700 Americans will receive a diagnosis of lung cancer every year with 164,440 of them expected to die of the disease, a mortality rate of over 90%. Survival rates for lung cancer have changed little over the past twenty-five years despite years of research, which is because most lung cancers are diagnosed at an advanced stage when curative treatment is no longer possible. Identifying lung cancer at the earliest stage is central to improving outcomes; even a small decrease in lung cancer mortality from effective screening of high-risk individuals would save thousands of lives each year. However, current screening techniques, such as chest radiology, sputum cytology and chest CT scanning are insensitive or non-specific and are not recommended for use by the American Cancer Institute. It is for problems such as the detection of lung cancer that an instrument for identifying molecules is desired.
It has been widely accepted that optical detection, or an “optical nose,” is the only approach that can meet the extreme demands for molecular detection. For ultimate performance, optical detection of molecules has to be performed in the so-called “molecular fingerprint region,” located in the 2 and 20 μm mid-IR spectral range. In this region, all the important gas molecules possess dense series of strong, narrow-band absorption lines corresponding to rotational-vibrational transitions. By measuring the mid-IR light absorption of a gas sample, basically all compounds can be identified and their individual concentrations can be measured.
Despite the need for such molecule detection, this substantial potential of mid-IR optical detection of molecules cannot be realized with any of the currently existing laser sources and detection techniques. Consequently, progress in many research areas of life-sciences has been held back since such areas depend on a fast and complete analysis of gas samples. For advanced molecular detection, novel mid-IR laser sources must be developed that can measure molecules in gases and vapors which, at atmospheric pressure, have spectral features which are typically a few 0.1 cm−1 wide. At low pressures, where Doppler broadening dominates, these features can be even sharper: <10−2 cm−1. In this case, single-longitudinal-mode (SLM) lasers are desirable. In addition to this high spectral resolution, a high species selectivity and full profiling capability requires a maximum coverage of the molecular fingerprint spectral region (2-20 μm, see HITRAN database). For real time analysis, the novel laser sources have to provide good wavelength agility (hundred thousands of distinct wavelengths per second). Moreover, multiple envisioned practical applications of the designed instrument, including use in environmental monitoring, geo-physical exploration, counterterrorism (explosive detection), food production, and biomedicine (breath analysis for early diagnostics of diseases) dictate necessity of extremely high detection sensitivities, up to parts per trillion which, in turn, requires high power (Watt level) and low-noise sources.
Even after four decades of laser and nonlinear optical research, ideal versatile sources capable of generating the mid-IR radiation required for the envisioned applications do not exist. Previous attempts to utilize mid-IR laser sources, such as lead salt diode lasers, quantum cascade lasers, and free-electron lasers, suffered from their insufficient output power or spectral controllability, resulting in an unacceptably low detection sensitivity, selectivity and speed. So far, the most promising sources of coherent radiation for mid-IR optical nose are optical parametric oscillators (OPOs) that are powerful solid state sources of broadly tunable coherent radiation, capable of covering the entire spectral range from the near UV to the mid IR, and can operate from continuous wave (CW) down to the femtosecond pulse durations. OPOs are capable of generating coherent radiation at wavelengths where lasers perform badly or are unavailable. In the molecular fingerprint region of the optical spectrum, 2-20 μm, where we lack broadly tunable lasers, similar to dye lasers in the visible or titanium-doped sapphire (Ti—S) in the near infrared, the OPOs play a particularly important role.
Conventional continuous wave (cw), especially diode and fiber laser pumped OPOs are promising of being capable to deliver highest spectral purity of the output radiation in a compact set-up. However, such OPOs are restricted to wavelengths shorter than 5 μm and require utilization of single (SRO) and/or double resonant cavities (DRO) for significant threshold reduction. This has the consequence that the length of the OPO cavity should be precisely stabilized to the pump laser wavelength by piezoelectric transducer (PZT) control system that make very problematic fast and broad mode-hope free tuning of the system required for envisioned applications.
Conventional pulsed OPOs and optical parametric generators (OPGs) are promising, although their spectra are either difficult to control (ns OPOs), or too broad (ultrashort pulsed OPOs), or the average power is too low (OPGs). It is noteworthy that, even when novel proposed coherent mid-IR sources will be developed, the detection of their radiation poses further challenges. Fast photodiodes, photomultipliers and CCD cameras, readily available in the visible and near-IR, do not exist in the mid-IR. Hence, highly effective signal enhancement techniques such as cavity ring-down spectroscopy (CRDS), multipass, frequency modulation, and photoacoustic enhancement techniques will require special attention to be successfully implemented in the mid-IR.